Physical vapour deposition by means of sputtering has become a standard technique to customise the properties of glass panes or other transparent materials. ‘Sputtering’ refers to the ballistic ejection of coating material atoms out of a target by means of positively charged ions of a heavy noble gas—usually argon—that are accelerated by an electric field towards a negatively charged target. The positive ions are formed by impact ionisation in the low pressure gas phase. The ejected atoms impinge with high kinetic energy on the substrate to be coated where they form a dense, well adhering coating.
The ionisation of the noble gas is confined close to the surface of the target by means of a magnetic field generated from within the target and exhibiting an arc shaped, closed loop tunnel at the surface of the target. During operation, electrons bounce back and forth along those magnetic field lines while drifting down the closed loop thereby increasing the impact ionisation of the noble gas atoms. A purple glowing closed loop ‘race track’ forms at the surface of the target.
It remains an engineering challenge when one wants to use cylindrical, rotating targets instead of the easier to implement planar stationary targets. When using the latter the coolant supply (the target must be cooled as the impact of the positive ions heats up the target) and electrical energy supply can be done to a fixed target assembly. When using rotating targets the coolant and electric supply must be rotational compliant while maintaining vacuum integrity. However, the benefits of succeeding in this challenge are worth the effort as rotatable targets carry much more usable target material stock than planar targets do. Also rotatable targets are less prone to arcing compared to their planar counterparts. These advantages are particularly appreciated in inline coaters wherein substrates pass by the elongated, cylindrical target in a direction perpendicular to the axis of the target. In order to maintain even coating thicknesses across the substrate a uniform sputter rate of target material is needed over the length of the target.
One of the engineering problems one is faced with is that the magnetic field generator must be contained in the target. The magnetic field generator—oriented towards the substrate to be coated—is usually held stationary while the cylindrical target rotates in front of it. High performance permanent magnets based on iron neodymium boron (Fe—Nd—B) or cobalt samarium (Co—Sm) alloys are used to generate the magnetic field. As the component of the magnetic field parallel to the surface of the target is determining the confinement of the electrons in the plasma it is important that this component is constant along the length of the tube. Unfortunately, the magnetic induction (in tesla) of this component normally drops with at least the second power of the distance to its generator and hence is very sensitive to the position of the magnetic field generator with respect to the target surface. The distance between the target surface and the magnetic field generator must therefore be well controlled as otherwise the plasma would show local variations in intensity that in their turn can lead to non-uniform coating profiles across the substrate. In what follows this will be called the ‘distance control problem’.
Most of the energy supplied to the target is converted into heat that is generated at the surface of the target. This heat must be carried away efficiently, as otherwise the target would heat up too much and the magnets of the field generator would loose their strength at higher temperatures leading to a ‘cooling problem’. Therefore a cooling circuit must be provided with a coolant that is normally water conditioned for the purpose. Unfortunately, the high performance magnets tend to corrode very fast and therefore they must be sealed from the coolant (‘the sealing problem’)
A number of solutions to the ‘cooling problem’ have been described in the prior-art. There is e.g. U.S. Pat. No. 5,262,032 that describes cooling of the magnets only. Or there is US 2005/0178662 that describes a system that provides for a radial exit of the coolant in the vicinity of the magnet.
A more comprehensive design attempt that seeks to solve at least two of the three problems mentioned above is U.S. Pat. No. 5,571,393 (Viratec thin films Inc.) wherein the magnet array is enclosed in a sealed magnet cavity part of a magnet housing made of two shells that are bolted together. The distance between target surface and magnet array is controlled through a pair of adjustable rollers mounted sideways of the magnet cavity and rolling against the inner side of the target. The coolant is fed from the one end of the magnetron through a central tube towards the other end and returns in axial direction in the gap between magnet housing and the target tube towards the one end. The disadvantage of this approach is that the coolant gets hotter and hotter as it nears the one end and hence leads to a gradient in temperature on the target and the magnet that on its turn leads to a gradient in deposition rate.
Another design can be found in WO 03/015124 (Bekaert Advanced Coatings). There the use of distance adjustment devices are described that set the distance between the target and the magnet array by locally deflecting the magnet field generator relative to a support structure. The support structure consists of the supply tube and a reinforcement element attached thereto. The element is filled with hard foam to increase its buoyancy in the coolant. The magnet array is sealed from the coolant by a magnet housing. The housing increases the stiffness of the magnet system thereby increasing the stresses on the support structure when the adjustment devices are used to set the distances between magnet array and target. Again the coolant flow is substantially axial from the one end of the target to the other end leading to a temperature gradient.
In a recent attempt to eliminate still water zones, bubble formation and improve the circulation of the coolant a series of US publications 2007/0089982, 2007/0089985, and 2007/0089986 describe the use of baffles to induce turbulence and of vanes to induce circulation into the cooling water. In addition the use of a rectangular shaped carrier tube to which the magnet array is attached is claimed for decreasing the bending of the magnet array.
WO 2005/005682 discloses a central body for mounting inside a rotatable target that incorporates—over and above the magnet array—means for bearing the target tube, rotating the tube, electrically contacting the tube and sealing the inside of the tube from coolant and vacuum. However, neither the magnet array is sealed from the coolant nor are adjustment devices provided to adapt the magnetic field. The use of this magnetron is limited to smaller scale installations.